[llvm] Forward-declare formatted_raw_ostream (NFC)

[llvm] Forward-declare formatted_raw_ostream (NFC)

Various *TargetStreamer.h need formatted_raw_ostream but rely on a
forward declaration of formatted_raw_ostream in MCStreamer.h.  This
patch adds forward declarations right in *TargetStreamer.h.

While we are at it, this patch removes the one in MCStreamer.h, where
it is unnecessary.